We've held off on adopting Slack because Hangouts Chat seemed promising. But here we are almost 3 years since it was introduced and there's still no integration with Gmail. Argh!
To add insult to injury, while we're stuck using crusty old Google Chat, (due to its integration with Gmail), Microsoft Teams is adding features regularly and has blown by Slack in terms of DAU.
Google Sites is even worse. "New" Google Sites was introduced in June of 2016. Since then, it's received very little attention in terms of ongoing development.
Meanwhile, companies like https://www.notion.so/ are completely eating their lunch (Notion has only 27 employees last time I checked). It's hard for me to believe that Google employees use Google Sites internally in its current state when there are so many better options out there.
I've lost all hope on Sites. I'm still hopeful that Google will make Hangouts Chat competitive with Slack and M$ Teams.
All that said, Gmail, Gcal, and Google Drive are still really really good. Even Hangouts Meet has improved significantly over the past year.